A new method of the reconstruction of focal spot and far-field focal spot measurement based on the diffraction grating is proposed, the validity of new method in expanding the measurement dynamic range and extracting the focal spot side lobe data is analyzed and the application condition is given in theory. According to the far-field spot measurement system available in laboratory and 1-D linear diffraction grating designed, power in the bucket (PIB) curve is taken as the measure of the validity of new method, the applicability of this new method is investigated by numerical simulation and experiment, in which the influence of the choice of the threshold on the results is discussed from the view of accurately calculating focal spot quality. The results are discussed and analyzed.